Regarding lasers, one big problem with using them in warfare (as lethal weapons) is their complexity. Guns are relatively simple things, and anyone trained in using them can fix a jam and whatnot without too much difficulty. Not to mention they're pretty sturdy, so it's not a big risk of having a problem with one. And should a gun be rendered unusable, so what? There's certainly plenty more available due to low construction costs.
Lasers are fare less simplistic in their operation, and any number of malfunctions could occur that could take an unreasonable amount of time to fix, if it's even fixable at all. Also note that powerful lasers require a pretty powerful cooling system, and that can make them heavy/bulky. Not exactly something I'd want to use as a soldier, though admittedly they're pretty badass.
